Typical materials requiring surface flatness at high level include a single-crystal silicon disk called a silicon wafer for producing semiconductor integrated circuits (IC, LSI). The surface of the silicon wafer should be flattened highly accurately in a process of producing IC. LSI etc., in order to provide reliable semiconductor connections for various coatings used in manufacturing the circuits. In the step of polishing finish, a polishing pad is generally stuck on a rotatable supporting disk called a platen, while a workpiece such as a semiconductor wafer is stuck on a polishing head. By movement of the two, a relative speed is generated between the platen and the polishing head while polishing slurry having abrasive grains is continuously supplied to the polishing pad, to effect polishing processing.
As techniques manufacturing a polyurethane foam, which is a material of a polishing pad, methods have been known, in one of which an organic solvent with a low boiling point such as fleon or methylene chloride is added and dispersed into a foam forming raw composition to foam a polymer by gasification thereof due to polymerization heat and in the other of which water is added and dispersed into a foam forming raw composition to foam a polymer with carbon dioxide generated by the reaction of water with an isocyanate group. Foams obtained by means of the methods have cells with the lower limit of an average diameter of 100 μm therein, thereby having encountered a difficulty obtaining a foam with more finer, uniform cells.
As manufacturing methods of polyurethane foam with fine cells therein, the following methods have been known:    (1) a method in which solvent soluble fine particles are dispersed into a polyurethane polymer, the polymer is molded into a predetermined shape and thereafter, the molded intermediate is immersed into a solvent in which the polyurethane polymer is not dissolved but the fine particles are dissolved to thereby dissolve and remove the fine particles and to form porous polyurethane resin, that is a foam (Patent Literature 1), and    (2) a method in which fine-cavity foams are dispersed into a polyurethane resin forming raw composition (Patent Literatures 2 and 3).
According to the method of Patent Literature 1, however, a great quantity of a solvent is required and a treatment of the solvent containing a fine particle forming material is further required, which entails a high cost. An obtained foam contains only open cells and cannot be used in application requiring rigidity, which imposes limitation on application. What's worse, a necessity arises even for a step of elution and a step of evaporating a solvent, which has leads to a problem of calling for a long time in fabrication of a molded product with a large thickness.
On the other hand, in the methods of Patent Literatures 2 and 3, since fine-cavity foams have a great tendency to float up in a raw polyurethane reaction liquid due to a difference in density, a difficulty is encountered manufacturing a uniform foam, the fine-cavity foams are comparatively expensive and a material of the fine-cavity foams remains in the foam in a product, thereby having resulted in problems of giving damage to a blade in cutting the foam and the like. Moreover, the fine cavity foams are easy to fly away, which requires a great cost in installing facilities for keeping a working environment in a good condition.
A polyurethane resin foam is employed as a polishing pad used in fabrication of a silicon substrate for manufacturing a semiconductor device or the like and fabrication of an electronic substrate. A high precision polishing is demanded to a polishing pad in company with a progress toward a higher compactness in formed circuitry, and a hardness or the like of a polishing pad (a polyurethane foam) is requested so as to be adapted for a kind and particle diameters of particles contained in a polishing slurry used in polishing. For example, a ceria-based slurry is larger in particle diameter than a silica-based slurry and in a case where a ceria-based slurry is employed as a polishing slurry, it requires a polishing pad with a higher hardness than in a case where a silica-based slurry.
As a method of solving the problems, a production method, or a so-called mechanical foaming method, has been disclosed of a polyurethane foam having uniform, fine cells and with a higher hardness than a polyurethane foam with the same density, produced without using a foreign material such as a chemically reactive foaming agent including water, a gasification expansible foaming agent including fleon, fine-cavity foams, a solvent soluble material or the like (Patent Literature 4). As a chain extender, 4,4′-methylenebis(o-chloroaniline)(MOCA) is preferably used from the viewpoint of a reactivity and a physical property and the like of an obtained polyurethane foam. However, in a case where MOCA is used, a problem has arisen that a cell diameter in a polyurethane resin foam is larger and easy to cause fluctuations in the diameter, exerting an adverse influence on polishing characteristics.
As polishing characteristics of a polishing pad, on the other hand, it is requested that a polished object is excellent in planarity and in-plane uniformity and a polishing speed is large. A planarity and in-plane uniformity of a polished object can be improved to some extent with a polishing layer higher in elastic modulus. A polishing speed can be bettered by increasing a holding quantity of a slurry on a foam with cells therein.
As a polishing pad satisfying the above characteristics, a proposal has been offered of a polishing pad made from a polyurethane resin foam (Patent Literatures 5 to 8). The polyurethane resin foam is produced by reacting an isocyanate polymer with a chain extender (a curing agent) and as chain extenders, 4,4′-methylenebis(o-chloroaniline) (hereinafter, referred to as MOCA) is preferably employed from the viewpoint of a reactivity and a physical property and the like of an obtained polyurethane foam.
Since MOCA contains chlorine in a molecule thereof, however, and has a demerit in an environmental aspect: generation of a harmful material such as dioxin in disposal as waste, there is a possibility that usage is limited or prohibited in the future. Hence, a desire has been built up for development of a polishing pad containing no halogen.
As a high-molecular polyol component of an isocyanate prepolymer, polyether (polytetramethylene glycol with a number-average molecular weight in the range of from 500 to 1600) or a polycarbonate is preferably employed from the viewpoint of hydrolysis resistance, elastic characteristic, abrasion resistance and the like.
After planarization of many of semiconductor wafers using one polishing pad, fine depressions and protrusions on a surface of the polishing pad are worn away, resulting in reduction in performance to feed a polishing agent (a slurry) onto a polishing surface of a semiconductor wafer, decrease in planarization speed on a wafer polishing surface and besides, degradation in planarization characteristic. Therefore, after planarization of a predetermined number of semiconductor wafers are effected, the surface of the polishing pad is necessary to be renewed and roughened (dressing) with a dresser. A predetermined time of dressing generates numberless fine depressions and protrusions on the surface of the polishing pad and the surface of the pad is transformed into a fluffy state.
A conventional polishing pad, however, has had a problem that a dressing speed is low in dressing and a dressing time is excessively consumed.
Patent Literature 1: JP 2-91279 A
Patent Literature 2: JPN 8-500622 A
Patent Literature 3: JP 2000-343412 A
Patent Literature 4: JP 3490431
Patent Literature 5: JP 3013105
Patent Literature 6: JP 3516874
Patent Literature 7: JP 2000-17252 A
Patent Literature 8: JP 3359629